As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased. Among such secondary batteries is a lithium secondary battery exhibiting high energy density and operating voltage and excellent preservation and service-life characteristics, which has been widely used as an energy source for various electronic products as well as mobile devices.
However, various kinds of combustible materials are contained in the lithium secondary battery. As a result, the lithium secondary battery may be heated or explode due to the overcharge of the battery, the overcurrent in the battery, or other external physical impact. That is, the safety of the lithium secondary battery is very low. Consequently, safety elements, such as a positive temperature coefficient (PTC) element and a protection circuit module (PCM), to effectively control an abnormal state of the lithium secondary battery, such as the overcharge of the lithium secondary battery or the overcurrent in the lithium secondary battery, are loaded on the lithium secondary battery in a state in which the safety elements are connected to a battery cell.
Generally, the PCM is connected to the battery cell via conductive nickel plates by welding or soldering. That is, the nickel plates are connected to connection terminals of the PCM by welding, and then the nickel plates are connected to corresponding electrode terminals of the battery cell by welding. In this way, the PCM is connected to the battery cell to manufacture a battery pack.
Specifically, a method of manufacturing the battery pack will be described with reference to FIG. 1. Nickel plates 10 are connected to cathode terminals and anode terminals of battery cells 20 by welding so that the battery cells 20 can be connected in series or in parallel to each other. Subsequently, the nickel plates 10 are electrically connected to connection terminals of a PCM 30 by welding to complete a battery pack. At this time, series or parallel electrical connection between the battery cells is achieved depending upon the connection structure of the nickel plates.
However, the above battery pack is manufactured through two steps, i.e. by electrically connecting the battery cells in series or in parallel to each other using the nickel plates and connecting the nickel plates to the PCM by welding. As a result, the structure and manufacturing process of the battery pack are complicated, the manufacturing costs of the battery pack are increased, and a defect rate of the battery pack is increased.
Therefore, there is a high necessity for a structure in which the number of welding times is minimized during electrical connection between the electrode terminals of the battery cells and the PCM.
To this end, a nickel plate of a PCM may be welded to a cathode terminal, which is an aluminum terminal, of a battery cells using a resistance welding rod (see FIG. 2). In this case, however, electric current generated from an anode of the welding rod 340 flows to a cathode of the nickel plate 320 via only the aluminum terminal 330 in a state in which the electric current does not reach the nickel plate 320 since the resistance of the aluminum terminal 330 is lower than that of the nickel plate 320. As a result, welding between the nickel plate 320 of the PCM and the aluminum terminal 330 of the battery cell through resistance welding may not be achieved, or welding coupling force therebetween may be low.
Therefore, there is a high necessity for a battery pack having a specific structure to achieve electrical connection between the nickel plate of the PCM and the aluminum terminal of the battery cell through resistance welding.
Meanwhile, a battery pack mounted in a laptop computer requires high power and large capacity. To this end, a conventional cylindrical battery pack including a plurality of cylindrical battery cells has generally been used. In recent years, however, the size of a laptop computer has been reduced, and therefore, there is a high necessity for a slim type battery pack.
Therefore, there is a high necessity for a technology in which pouch-shaped battery cells are used to manufacture a slim type battery pack, thereby increasing capacity of the battery pack, and electrode terminals of the battery cells are directly electrically connected to connection terminals of a PCM, thereby simplifying a process of manufacturing a battery pack and minimizing overall size of the battery pack.